Components used in high stress, high temperature applications ("high intensity" components) typically are provided with protective coatings to prevent material oxidation and hot corrosion during service. One type of protective coating for high intensity components, such as gas turbines, is an overlay coating. A popular overlay coating has a chemical composition of "MCrAlY"--where "M" is nickel, cobalt, or both, Cr is chromium, Al is aluminum, and Y is yttrium.
Certain types of components are subject to particularly high stress and high temperature conditions during use (hereinafter called "super high intensity" components). Examples of super high intensity components are jet engine parts and turbo-superchargers. In order to withstand the extreme service conditions, super high intensity components typically are made of a base material known as a "superalloy." Superalloys exhibit high temperature mechanical integrity with an unusual degree of oxidation and creep resistance. Popular protective coatings for super high intensity superalloy components are thermal barrier coatings (TBCs). TBCs maintain the temperature of the superalloy substrate at an acceptable operating level during service.
Unfortunately, protective coatings for high intensity and super high intensity components are not, themselves, immune to material degradation. One cause of material degradation in MCrAlY protective coatings is the diffusion of constituents from the coating, particularly the diffusion of aluminum. As seen from the following, the problem is most acute in thermal barrier coatings.
TBC's are multi-layered protective coatings. Basically, the innermost layer--next to the superalloy substrate--is a "bondcoat," which generally is made of MCrAlY. Just outside of the bondcoat is a middle zone, which is a very thin layer of aluminum oxide (Al.sub.2 O.sub.3). The outermost layer typically is an yttria stabilized zirconia ceramic ("YSZ").
The aluminum oxide layer in a TBC tends to thicken during service. One cause of such thickening apparently is the diffusion of aluminum from the MCrAlY bondcoat to the aluminum oxide layer. As the aluminum oxide layer thickens, thermally induced cracking of the TBC occurs. The loss of aluminum from the MCrAlY bondcoat, and thickening of the aluminum oxide layer, are major causes of material degradation in TBCs. One way to reduce the loss of protectivity of such coatings is to suppress the diffusion of constituents from the coating into the substrate or--where the coating is a TBC--between the layers of the coating.
Some have attempted to improve the performance of MCrAlY coatings by adding high atomic weight elements, such as rhenium (Re), as an integral component of such coatings. Overlay coatings comprised of MCrAlY laced with rhenium are reported to have increased oxidation resistance and decreased thermal and material degradation.
Although some success has been reported when Re is used as an integral additive in MCrAlY overlay coatings, the use of Re as an additive to the coating necessarily results in random distribution of Re atoms throughout the coating. Some of the constituent aluminum atoms in the coating necessarily will diffuse past such randomly dispersed Re atoms and out of the coating.
A more effective means to suppress the diffusion of constituents from protective coatings, particularly a means to suppress the diffusion of aluminum from MCrAlY coatings, would prolong the useful lifetime of such coatings.